Oil‐Impregnated Nanoporous Oxide Layer for Corrosion Protection with Self‐Healing

The major drawback of current passivation techniques for preventing corrosion is the lack of ability to withstand any external damages or local defects. In this study, oil‐impregnated nanoporous anodic aluminum oxide (AAO) layers are investigated to overcome such limitations and thus advance corrosion protection. By completely filling hydrophobized nanopores with oil via a solvent exchange method, a highly water‐repellent surface that prevents the penetration of corrosive media into the AAO layer and hence the corrosion of aluminum is achieved. The impregnation of oil into the hydrophobic nanoporous AAO layer enhances the corrosion resistance of an AAO layer by two and four orders of magnitude compared to that of a hydrophobic (i.e., air‐entrained) and a bare (hydrophilic) AAO, respectively. In the presence of local defects, the oil impregnated within the hydrophobic nanoporous AAO layer naturally permeates into the defects and ultimately inhibits the exposure of the aluminum surface to corrosive media. Whereas the corrosion current density of the air‐entrained hydrophobic AAO layer increases by more than 30 times after cracks, that of the oil‐impregnated AAO layer increases by no more than 4 times, showing superior anticorrosion property even after there are cracks, owing to the effective self‐healing capability.     

Nanocontainer‐Based Anticorrosive Coatings: Effect of the Container Size on the Self‐Healing Performance

Organic coatings based on inhibitor loaded inorganic containers for smart corrosion inhibition are presented. The overall coating performance is strongly influenced by the containers as well as their inhibitor capacity, compatibility with the coating matrix, and size. The important effect of container size is described for the first time in this work by investigating two types of mesoporous silica containers of different diameters: 80 and 700 nm. The coating physical properties (thickness and adhesion) are comparable for both container types. In contrast, the coating barrier properties are strongly influenced by the container size as assessed with electrochemical impedance spectroscopy (EIS). The incorporation of bigger containers reduces the coating resistance by a factor of two. Surprisingly, despite the similar amounts (20 wt%) of loaded inhibitor (2‐mercaptobenzothiazole), different active inhibition ability is detected with the scanning vibrating electrode technique (SVET). Therefore, it is found that coatings with smaller containers exhibit better self‐healing performance.     

Self‐Healing Silk Fibroin‐Based Hydrogel for Bone Regeneration: Dynamic Metal‐Ligand Self‐Assembly Approach

Despite advances in the development of silk fibroin (SF)‐based hydrogels, current methods for SF gelation show significant limitations such as lack of reversible crosslinking, use of nonphysiological conditions, and difficulties in controlling gelation time. In the present study, a strategy based on dynamic metal‐ligand coordination chemistry is developed to assemble SF‐based hydrogel under physiological conditions between SF microfibers (mSF) and a polysaccharide binder. The presented SF‐based hydrogel exhibits shear‐thinning and autonomous self‐healing properties, thereby enabling the filling of irregularly shaped tissue defects without gel fragmentation. A biomineralization approach is used to generate calcium phosphate‐coated mSF, which is chelated by bisphosphonate ligands of the binder to form reversible crosslinkages. Robust dually crosslinked (DC) hydrogel is obtained through photopolymerization of acrylamide groups of the binder. DC SF‐based hydrogel supports stem cell proliferation in vitro and accelerates bone regeneration in cranial critical size defects without any additional morphogenes delivered. The developed self‐healing and photopolymerizable SF‐based hydrogel possesses significant potential for bone regeneration application with the advantages of injectability and fit‐to‐shape molding.     

Developments and Challenges in Self‐Healing Antifouling Materials

Self‐healing antifouling materials have gained rapidly increasing interest over the past decade and have been studied and used in a rapidly increasing range of applications. Recent developments and challenges in self‐healing antifouling materials are summarized in four sections: first, the different mechanisms for both antifouling and self‐healing are briefly discussed. Second, three main categories of self‐healing antifouling materials based on surface replenishing and dynamic covalent and noncovalent interactions are discussed, with a focus on the preparation, characterization, and central characteristics of different self‐healing antifouling materials. Third, different types of potential applications of self‐healing antifouling materials are summarized, such as injectable hydrogels and oil/water separations. Finally, a summary of future development of the field is provided, and a number of critical limitations that are still outstanding are highlighted.     

Novel Biocompatible Polysaccharide‐Based Self‐Healing Hydrogel

A novel biocompatible polysaccharide‐based self‐healing hydrogel, CEC‐l‐OSA‐l‐ADH hydrogel (“l” means “linked‐by”), is developed by exploiting the dynamic reaction of N‐carboxyethyl chitosan (CEC) and adipic acid dihydrazide (ADH) with oxidized sodium alginate (OSA). The self‐healing ability, as demonstrated by rheological recovery, macroscopic observation, and beam‐shaped strain compression measurement, is attributed to the coexistence of dynamic imine and acylhydrazone bonds in the hydrogel networks. The CEC‐l‐OSA‐l‐ADH hydrogel shows excellent self‐healing ability under physiological conditions with a high healing efficiency (up to 95%) without need for any external stimuli. In addition, the CEC‐l‐OSA‐l‐ADH hydrogel exhibits good cytocompatibility and cell release as demonstrated by three‐dimensional cell encapsulation. With these superior properties, the developed hydrogel holds great potential for applications in various biomedical fields, e.g., as cell or drug delivery carriers.     

Acylhydrazones as Reversible Covalent Crosslinkers for Self‐Healing Polymers

The utilization of dynamic covalent and noncovalent bonds in polymeric materials offers the possibility to regenerate mechanical damage, inflicted on the material, and is therefore of great interest in the field of self‐healing materials. For the design of a new class of self‐healing materials, methacrylate containing copolymers with acylhydrazones as reversible covalent crosslinkers are utilized. The self‐healing polymer networks are obtained by a bulk polymerization of an acylhydrazone crosslinker and commercially available methacrylates as comonomers to fine‐tune the T_g of the systems. The influence of the amount of acylhydrazone crosslinker and the self‐healing behavior of the polymers is studied in detail. Furthermore, the basic healing mechanism and the corresponding mechanical properties are analyzed.     

Self‐Healing and Stretchable 3D‐Printed Organic Thermoelectrics

With the advent of flexible and wearable electronics and sensors, there is an urgent need to develop energy‐harvesting solutions that are compatible with such wearables. However, many of the proposed energy‐harvesting solutions lack the necessary mechanical properties, which make them susceptible to damage by repetitive and continuous mechanical stresses, leading to serious degradation in device performance. Developing new energy materials that possess high deformability and self‐healability is essential to realize self‐powered devices. Herein, a thermoelectric ternary composite is demonstrated that possesses both self‐healing and stretchable properties produced via 3D‐printing method. The ternary composite films provide stable thermoelectric performance during viscoelastic deformation, up to 35% tensile strain. Importantly, after being completely severed by cutting, the composite films autonomously recover their thermoelectric properties with a rapid response time of around one second. Using this self‐healable and solution‐processable composite, 3D‐printed thermoelectric generators are fabricated, which retain above 85% of their initial power output, even after repetitive cutting and self‐healing. This approach represents a significant step in achieving damage‐free and truly wearable 3D‐printed organic thermoelectrics.     

Humidity‐Triggered Self‐Healing of Microporous Polyelectrolyte Multilayer Coatings for Hydrophobic Drug Delivery

Layer‐by‐layer (LbL) self‐assemblies have inherent potential as dynamic coatings because of the sensitivity of their building blocks to external stimuli. Here, humidity serves as a feasible trigger to activate the self‐healing of a microporous polyethylenimine/poly(acrylic acid) multilayer film. Microporous structures within the polyelectrolyte multilayer (PEM) film are created by acid treatment, followed by freeze‐drying to remove water. The self‐healing of these micropores can be triggered at 100% relative humidity, under which condition the mobility of the polyelectrolytes is activated. Based on this, a facile and versatile method is suggested for directly integrating hydrophobic drugs into PEM films for surface‐mediated drug delivery. The high porosity of microporous film enables the highest loading (≈303.5 μg cm^?2 for a 15‐bilayered film) of triclosan to be a one‐shot process via wicking action and subsequent solvent removal, thus dramatically streamlining the processes and reducing complexities compared to the existing LbL strategies. The self‐healing of a drug‐loaded microporous PEM film significantly reduces the diffusion coefficient of triclosan, which is favorable for the long‐term sustained release of the drug. The dynamic properties of this polymeric coating provide great potential for its use as a delivery platform for hydrophobic drugs in a wide variety of biomedical applications.     

Self‐Healing Graphene Oxide Based Functional Architectures Triggered by Moisture

Self‐healing materials are capable of spontaneously repairing themselves at damaging sites without additional adhesives. They are important functional materials with wide applications in actuators, shape memorizing materials, smart coatings, and medical treatments, etc. Herein, this study reports the self‐healing of graphene oxide (GO) functional architectures and devices with the assistance of moisture. These GO architectures can completely restore their mechanical‐performance (e.g., compressibility, flexibility, and strength) after healing their broken sites using a little amount of water moisture. On the basis of this effective moisture‐triggered self‐healing process, this study develops GO smart actuators (e.g., bendable actuator, biomimetic walker, rotatable fiber motor) and sensors with self‐healing ability. This work provides a new pathway for the development of self‐healing materials for their applications in multidimensional spaces and functional devices.     

Accelerated Self‐Healing Via Ternary Interpenetrating Microvascular Networks

Self‐healing materials with dual interpenetrating microvascular networks enable two‐part healing chemistries and repeated healing of damage in a localized region. 1 However, due to slow healing kinetics, multiple days are required between damage events to recover mechanical performance under ambient conditions. By directly writing a third interdigitated microvascular network within these epoxy coating/substrate architectures to enable in situ thermal regulation, the characteristic healing time is reduced by an order of magnitude. Specifically, this third network provides a conduit for circulating a temperature‐controlled fluid that rapidly heats the locally damaged region leading to a sharp reduction in the time required for mechanical property restoration.     

Self‐Healing Materials for Energy‐Storage Devices

The booming development of electronics, electric vehicles, and grid storage stations has led to a high demand for advanced energy‐storage devices (ESDs) and accompanied attention to their reliability under various circumstances. Self‐healing is the ability of an organism to repair damage and restore function through its own internal vitality. Inspired by this, brilliant designs have emerged in recent years using self‐healing materials to significantly improve the lifespan, durability, and safety of ESDs. Extrinsic and intrinsic self‐healing materials and their working principles are first introduced. Then, the application of self‐healing materials in ESDs according to their self‐healing chemistry, including hydrogen bonds, electrostatic interactions, and borate ester bonds, are described in detail. Based on these, critical challenges and important future directions of self‐healing ESDs are discussed.     

Tacky Elastomers to Enable Tear‐Resistant and Autonomous Self‐Healing Semiconductor Composites

Mechanical failure of π‐conjugated polymer thin films is unavoidable under cyclic loading conditions, due to intrinsic defects and poor resistance to crack propagation. Here, the first tear‐resistant and room‐temperature self‐healable semiconducting composite is presented, consisting of conjugated polymers and butyl rubber elastomers. This new composite displays both a record‐low elastic modulus (<1 MPa) and ultrahigh deformability with fracture strain above 800%. More importantly, failure behavior is not sensitive to precut notches under deformation. Autonomous self‐healing at room temperature, both mechanical and electronic, is demonstrated through the physical contact of two separate films. The composite film also shows device stability in the ambient environment over 5 months due to much‐improved barrier property to both oxygen and water. Butyl rubber is broadly applicable to various p‐type and n‐type semiconducting polymers for fabricating self‐healable electronics to provide new resilient electronics that mimic the tear resistance and healable property of human skin.     

Self‐Healing of an Epoxy Resin Using Scandium(III) Triflate as a Catalytic Curing Agent

A novel Lewis acid‐catalysed self‐healing system is investigated for implementation into epoxy‐based fibre reinforced polymer (FRP) composite materials. The catalyst, scandium(III) triflate, is selected using a qualitative approach and subsequently embedded with pre‐synthesised epoxy‐solvent loaded microcapsules, into an epoxy resin. Healing is initiated when microcapsules are ruptured at the onset of crack propagation. The epoxy monomer healing agent contained within actively undergoes ring‐opening polymerisation (ROP) on contact with the locally dispersed catalyst, forming a new polymer to bridge the two fractured crack planes. Self‐healing performance is quantified using a tapered double cantilever beam (TDCB) test specimen and the effects of microcapsule content and healing temperature and time are all independently considered. As an initial ‘proof of concept’ study, results show that a material recovery value of greater than 80% fracture strength is achieved for this novel Lewis acid‐catalysed self‐healing epoxy resin.     

Autonomous Self‐Healing of Epoxy Thermosets with Thiol‐Isocyanate Chemistry

Thiol‐isocyanate chemistry, combined with a dual capsule strategy, is used for the development of extrinsic self‐healing epoxy materials. It is shown that the amine groups present in the matrix both serve as a catalyst for the addition reaction between a thiol and an isocyanate and as a way to covalently link the healed network structure to the surrounding resin. The tapered double cantilever beam (TDCB) geometry is used for evaluating the recovery of the fracture toughness at room temperature after different healing times. Using manual injection of the healing agents into the crack, a healing efficiency up to 130% is obtained for the EPIKOTE 828/DETA epoxy material. On the other hand, when two types of microcapsules, one containing a tetrathiol reagent and the other a low toxic isocyanate reagent, are incorporated into this epoxy thermoset (20 wt%), a recovery of more than 50% is reached. The influence of parameters such as the amount and core content of the microcapsules on the healing efficiency is investigated. Furthermore, the thiol‐isocyanate chemistry is also tested for an industrial cold‐curing epoxy resin (RIM 135/RIMH 137).     

Transparent Self‐Healing Polymers Based on Encapsulated Plasticizers in a Thermoplastic Matrix

A self‐healing approach for optically transparent thermoplastic polymers, based on plasticizer‐induced solvent welding, is reported. For the specific system investigated, dibutylphthalate (DBP) filled urea‐formaldehyde capsules are dispersed in a polymethylmethacrylate (PMMA) matrix. Upon a damage event, DBP is released into the crack, and locally plasticizes and swells the polymer, enabling it to remend. Two challenges are addressed to maintain optical transparency: minimization of light scatter from the capsules in the polymer matrix and minimization of light scatter from the healed polymer. PMMA films containing DBP capsules have good transmissive properties as a result of the close index match between PMMA and DBP. The transmission properties are better than, for example, when DBP capsules are dispersed into a poorly index matched matrix, such as polystyrene. In the DBP PMMA system, the healed material is inherently index matched to the polymer matrix and thus the polymer's original optical properties are largely restored. Self‐healing using both small capsules, 1.5 μm in diameter, and large capsules, 75 μm in diameter is demonstrated. Smaller capsules are particularly important for thin polymer films which are not thick enough to hold the larger capsules. Polymer films with smaller capsules also have very good transmission properties due to a minimization of light scattering by the small size of the capsules. Large capsules enable healing of larger damage events, but do inherently result in some light scattering. This plasticizer‐based approach to self‐healing is shown to enable recovery of the protective properties and a portion of the mechanical properties of a polymeric film.     

Delivery of Two‐Part Self‐Healing Chemistry via Microvascular Networks

Multiple healing cycles of a single crack in a brittle polymer coating are achieved by microvascular delivery of a two‐part, epoxy‐based self‐healing chemistry. Epoxy resin and amine‐based curing agents are transported to the crack plane through two sets of independent vascular networks embedded within a ductile polymer substrate beneath the coating. The two reactive components remain isolated and stable in the vascular networks until crack formation occurs in the coating under a mechanical load. Both healing components are wicked by capillary forces into the crack plane, where they react and effectively bond the crack faces closed. Healing efficiencies of over 60% are achieved for up to 16 intermittent healing cycles of a single crack, which represents a significant improvement over systems in which a single monomeric healing agent is delivered.